public static String getSmallestAndLargest(String s, int k) {
    String smallest = "";
    String largest = "";

    // Complete the function
    // 'smallest' must be the lexicographically smallest substring of length 'k'
    // 'largest' must be the lexicographically largest substring of length 'k'
    java.util.ArrayList<String> substr = new java.util.ArrayList<>(); // Empty list
    int i=0;

    while(i+k<=s.length()){
    substr.add(s.substring(i, i+k));
    i++;
    }
     java.util.Collections.sort(substr);
    smallest = substr.get(0);
    largest = substr.get(substr.size()-1);
    return smallest + "\n" + largest;
}